Objective Nuchal translucency (NT) seen ultrasonographically at 11-14 weeks' gestation is a sensitive marker for Down syndrome. Despite its important role for Down syndrome screening, its use is still considered controversial due to high false-positive rates. We speculated that progesterone could lead to abnormal blood flow patterns and, subsequently, to increased NT. Our primary endpoint was to evaluate the effects of exogenous progesterone on NT thickness compared to controls. The secondary endpoint was to evaluate these effects in a subgroup at low risk for fetal aneuploidies, identifying the strongest factors influencing NT variation. The tertiary endpoint was to evaluate, within the treatment group, if there is any difference in NT according to the type of progesterone administered, route of administration, and dose regimen. Study Design All women who came to measure NT at 11-14 weeks' gestation (crown-rump length between 45-84 mm) were considered eligible. We divided patients into 2 groups: women receiving exogenous progesterone and controls. Afterwards, 3 NT scans were performed for each case, and the largest value, accurate to 2 decimal points, was recorded. Results In all, 3716 women were enrolled and analyzed. In a crude analysis, NT ( P P P P Conclusion Exogenous progesterone increases NT.